Quick answer: unfortunately, there is no way to isolate the numbering parts so that you can “decorate them”.
However I found a very dirty trick to get “chapter x section y” (without the commas) in cross-reference. But this dramatically impacts the heading numbering which becomes:
Chapter 1 Heading for chapter one
Section 1 First section
Section 2 Second section
Chapter 2 Heading for chapter two
See Chapter 1 Section 2 for details
Chapter Numbering is customised with various Separator Before and Show sublevels is set to 1 otherwise it does not work. Note also you can’t change case of the separator when it is inserted as a cross-reference. The separator itself becomes present in the heading and the TOC too.
When you insert a cross-reference to a heading, use Number (full context) in Insert reference to instead of the usual Chapter.
It may be possible to improve on that (removing the separators from headings and TOC) by assigning a double numbering to your headings: the traditional
Numbering to keep maximum versatility on displayed numbering and to keep TOC auto construction, plus a second hidden “decorated” numbering as above for use with cross-references. This would be implemented with a dedicated list style. I have not explored this path but, beware, it will be extremely tricky to avoid losing sync with chapter numbering. It is also tricky because any paragraph can have only oone number assigned to it. Therefore, it is likely that you’ll need some dummy hidden paragraph after any Heading n.